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ENG
If an alarm is active on compressor 3, the recalculated activation
thresholds are 10, 30, 70 %.
C2
10%
30%
Activation of the compressors and load stages may be:
•
Grouped (CpppCppp): fi rst all the load stages are activated on one
compressor before starting the next one
•
Balanced (CCpppppp): fi rst all the compressors are started at minimum
capacity and then the corresponding load stages are activated, one for
each compressor, in sequence.
6.4.3
Rotation with modulation devices
pRack PR300T can also manage compressor rotation when a capacity
modulation device is fi tted (inverter, Digital Scroll™ or continuous control).
The type of modulating device is selected and the corresponding
parameters set during the start-up procedure or in main menu branch
C.a.f/C.b.f and C.a.g/C.b.g
The modulating device is always the fi rst to start and the last to stop
irrespective of the type of rotation, the other devices start or stop
according to the type of rotation selected.
Note: The compressor with modulation device is also assumed to
be the fi rst.
The trend in capacity delivered by the modulation device depends on
the capacity of the compressor with the modulating device compared to
the other compressors available.
Three cases can be identifi ed:
•
compressors all with the same capacity and range of capacity variation
of the modulating device greater than or equal to the capacity of the
compressors
•
compressors all with the same capacity and range of capacity variation
of the modulating device less than the capacity of the compressors
•
compressors with diff erent capacities
In the fi rst case, the modulating device manages to continuously cover
the range of variation of the control request, while in the second case
some discontinuous variations remain. The behaviour in the third case
varies according to the capacities involved, and in any case refl ects one
of the two previous cases.
To confi gure the compressor capacity when an inverter is used, the
minimum and maximum operating frequencies need to be set relating
to the minimum and maximum value of the analogue output and the
rated capacity delivered at rated frequency (50 Hz), so that the pRack
PR300T software can calculate the capacity the compressor can deliver
with the inverter and use this value for control. In addition, for inverters
the variation in capacity delivered can be limited by setting the increase
and decrease times. If these times have already been confi gured on the
inverter, the higher time set has priority.
Example 1: range of modulating device capacity variation higher than
the capacity of the compressors:
Two compressors without capacity control, with the same capacity, 20
kW each, modulating device with variable capacity between 30 and 60
kW. The fi gure shows the trend when the request sent by the controller
increases and then decreases continuously between 0 and 100 %.
Copyright HPL March 2021
pRack PR300T +0300018EN rel. 1.5 - 02.05.2017
C4
70%
Regulation request
Fig. 6.o
It can be seen that the capacity delivered exactly follows the required
capacity, except when below the minimum capacity of the modulating
device.
Actual capacity (kW)
100
80
60
40
20
Regulation request
Inverter capacity
Example 2: range of modulating device capacity variation lower
than the capacity of the compressors: two compressors without
capacity control, with the same capacity, 30 kW each, modulating device
with variable capacity between 20 and 40 kW.
It can be seen that the capacity delivered does not exactly follow the
required capacity, rather acts in steps, so as to avoid swings.
Actual capacity (kW)
100
80
60
40
20
Regulation request
Inverter capacity
Example 3: range of modulating device capacity variation in
between the capacity of the compressors, all diff erent sizes: two
compressors without capacity control, capacities 15 kW and 25 kW,
modulating device with variable capacity between 10 and 30 kW.
Actual capacity (kW)
70
50
30
10
Regulation request
Inverter capacity
36
Page 85 of 169
Step compressors capacity
Total capacity
Fig. 6.p
Step compressors capacity
Total capacity
Fig. 6.q
Step compressors capacity
Total capacity
Fig. 6.r
P4367 Rev 01
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